There have been several innovations established in the last few years across all of the solar cell technologies that are geared towards reducing the cost of implementation of solar energy. These include newer architectures with higher efficiencies, refined material usage, lower cost materials, improved durability and higher throughputs. Much more rapid declines in the costs of solar power will in all likelihood require flexible modules that are not addressable using the market dominant crystalline silicon materials.
The perovskite solar cell is a relatively newer entrant into the solar photovoltaic technologies and has seen vast improvement in the power conversion efficiencies in a very short period of time. In particular, since the first report of perovskite solar cells in 2012, the technology has evolved to over 20% certified power-conversion efficiency. The cells are based on organometal halide perovskite materials characterized by high extinction coefficients and carrier mobilities. The perovskite structure is generally represented by the formula ABX3 and in the case of the organometal halide the A site refers to an organic group, B represents a metal such as lead (Pb), and X is a halide group such as iodine (I), chlorine (Cl), or bromine (Br).
Perovskite solar cells provide ease of manufacturing, use of common materials, and respectable efficiencies. More specifically, these solar cells combine the crystallinity and high charge-transfer found in inorganic semiconductors with the cost-effective low-temperature solvent-based manufacturing of organic solar cells. Additionally, unlike traditional semiconductor solar cells, the perovskite solar cell is amenable to changes in the atoms of its crystal structure. This opens up many possibilities in tuning band gaps, using different cell configurations, and experimenting in processing techniques.
Although there are many different variations in perovskite solar cell technology, the fundamental concepts of semiconductors devices apply to them all. Larger crystal sizes lead to fewer grain boundaries and enhanced charge-transfer with longer charge-carrier diffusion lengths. Grain boundaries have been known to introduce allowed energy levels in the band gap of a semiconductor and act as recombination centers. In other semiconductor technologies, the grain size can be increased by sintering the semiconductor at high temperatures.
However, because of the instability of the methylammonium lead iodide (CH3NH3PbI3) perovskite structure, annealing of perovskite materials is limited to temperatures <150° C. Above this temperature, the methylammonium iodide (MAI) begins to evaporate from the cell and the CH3NH3PbI3 decomposes into lead iodide (PbI2) and MAI. Additionally, when annealing at high temperatures, surface coverage is often reduced, resulting in the formation of perovskite islands due to agglomeration. More specifically, it has been demonstrated that as the annealing temperature increases, the number of pores in the final film decreases, but the size of the pores increases and the morphology transitions from a continuous layer into discrete islands of perovskite. An active layer morphology composed of discrete islands can create multiple shunting pathways by exposing the underlying contact and thus limiting performance. For at least these reasons, to date, the sintering techniques of other semiconductor technologies have not been explored as a viable alternative for the recrystallization of the unstable CH3NH3PbI3 material in connection with perovskite formation.
Other methods for creating larger perovskite crystals and improving the surface coverage of perovskite films is therefore important for optimizing device performance and has been a topic of discussion in perovskite solar cell research. Initially researchers improved the crystal formation utilizing a two-step sequential deposition of PbI2 followed by MAI. Others improved on that method by showing that heating the substrates prior to spin coating the PbI2 solution resulted in better surface coverage and pore filling of the perovskite formed after dipping the films in MAI solution. However, this two-step method (PbI2 spin coating followed by a MAI dip coat) is not ideal for roll-to-roll manufacturing. A more expedient one-step deposition of CH3NH3PbI3 has also recently been advanced by using solvent-solvent extraction techniques. These techniques utilize low boiling point solvents such as diethyl ether to remove high boiling point solvents such as Dimethyl sulfoxide (DMSO) or Dimethylformamide (DMF) from the perovskite films after spin coating. While these advances are significant in improving the morphology of perovskite thin films and device performance, they are difficult to scale.
In addition to the morphology issues, the CH3NH3PbI3 material of the perovskite solar cells can be susceptible to degradation from humidity, UV, and temperature. While a simple solution would be to hermetically seal the device, the same degradation mechanisms also limit the production of the perovskite layer to well controlled environs. As such, hermetically sealing the device does not solve the problem of manufacturing in ambient environments. Furthermore, the added sealing technologies and processing limitations add costs to the CH3NH3PbI3 perovskite solar cell that can offset the opportunities of these low priced materials.
Notwithstanding the issues discussed above, recently, there has been increasing interest among the research community in depositing perovskite films under ambient conditions. For example, some researchers have demonstrated that the film deposition under low humidity conditions and post-treatment of the films under high humidity conditions yields better crystallinity and performance. Others have investigated the role of moisture exposure during the perovskite film fabrication. Their results conclude that the moisture exposure enhanced the open-circuit voltage. Further researchers have studied the humidity induced grain boundaries in perovskite films where prolonged exposure of the film to high humidity conditions created additional grain boundaries. More recently, air stable and high efficiency perovskite cells with dissolving HCl gas in MAI, PbI2 and DMF solution were reported. Even though the perovskite film was formed with overnight drying at room temperature, the fabrication process was carried out in a nitrogen filled glove box over a prolonged time for drying, which likely is not a viable option for cost-effective large scale manufacturing.
To date, the role of alkyl halides in the crystal formation, solvent and solute interaction, and influence on film surface morphology has not been explored with photonic annealing techniques (such as intense pulsed light (IPL)). IPL heats a thin film using light energy, but the wavelengths span from the UV to the visible (vis) into the near infrared (NIR) and the duration of the pulse is on the order of a millisecond.
Accordingly, a heat treatment technique that can create a dense layer of perovskite particles with enlarged crystal sizes, and particularly a heat treatment technique for the production of perovskite films under ambient conditions, would be both highly desirable and beneficial. Likewise, a single-step perovskite deposition process under low humidity conditions would also be desirable.